nihav-core/src/io/bitreader.rs

   1 //! Bitstream reader functionality.
   2 //!
   3 //! Bitstream reader operates on `&[u8]` and allows to read bits from the slice in different modes.
   4 //!
   5 //! # Examples
   6 //!
   7 //! Reading 17 bits from a bitstream:
   8 //! ```
   9 //! use nihav_core::io::bitreader::{BitReader,BitReaderMode};
  10 //!
  11 //! # use nihav_core::io::bitreader::BitReaderResult;
  12 //! # fn foo() -> BitReaderResult<u32> {
  13 //! let bits: [u8; 4] = [ 42, 43, 44, 45 ];
  14 //! let mut br = BitReader::new(&bits, BitReaderMode::BE);
  15 //! let value = br.read(17)?;
  16 //! # Ok(value)
  17 //! # }
  18 //! ```
  19 //!
  20 //! Reading some amount of bits and checking how many bits are left:
  21 //! ```
  22 //! use nihav_core::io::bitreader::{BitReader,BitReaderMode};
  23 //!
  24 //! # use nihav_core::io::bitreader::BitReaderResult;
  25 //! # fn foo() -> BitReaderResult<()> {
  26 //! let bits: [u8; 4] = [ 42, 43, 44, 45 ];
  27 //! let mut br = BitReader::new(&bits, BitReaderMode::BE);
  28 //! let num_skip_bits = br.read(3)?;
  29 //! br.skip(num_skip_bits)?;
  30 //! println!("Now there are {} bits left to read.", br.left());
  31 //! # Ok(())
  32 //! # }
  33 //! ```
  34
  35
  36
  37 /// Bitstream reading modes.
  38 #[derive(Debug,Clone,Copy)]
  39 pub enum BitReaderMode {
  40     /// The stream is big endian MSB first.
  41     BE,
  42     /// The stream is little endian LSB first.
  43     LE,
  44     /// The stream is packed into 16-bit little-endian words MSB first.
  45     LE16MSB,
  46     /// The stream is packed into 32-bit little-endian words MSB first.
  47     LE32MSB,
  48 }
  49
  50 /// A list specifying general bitstream reading errors.
  51 #[derive(Debug,Clone,Copy)]
  52 pub enum BitReaderError {
  53     /// The reader is at the end of bitstream.
  54     BitstreamEnd,
  55     /// The caller tried to read too many bits at once (e.g. 128).
  56     TooManyBitsRequested,
  57     /// Some argument is invalid.
  58     InvalidValue,
  59 }
  60
  61 use self::BitReaderError::*;
  62
  63 /// A specialised `Result` type for bitstream operations.
  64 pub type BitReaderResult<T> = Result<T, BitReaderError>;
  65
  66 /// Bitstream reader.
  67 #[derive(Debug,Clone)]
  68 pub struct BitReader<'a> {
  69     cache: u64,
  70     bits:  u8,
  71     pos:   usize,
  72     src:   &'a [u8],
  73     mode:  BitReaderMode,
  74 }
  75
  76 #[allow(clippy::identity_op)]
  77 impl<'a> BitReader<'a> {
  78
  79     /// Constructs a new instance of bitstream reader.
  80     ///
  81     /// # Examples
  82     ///
  83     /// ```
  84     /// use nihav_core::io::bitreader::{BitReader,BitReaderMode};
  85     ///
  86     /// let bits: [u8; 4] = [ 42, 43, 44, 45 ];
  87     /// let mut br = BitReader::new(&bits, BitReaderMode::BE);
  88     /// ```
  89     pub fn new(src: &'a [u8], mode: BitReaderMode) -> Self {
  90         BitReader{ cache: 0, pos: 0, bits: 0, src, mode }
  91     }
  92
  93     /// Reports the current bit position in the bitstream (usually simply the number of bits read so far).
  94     pub fn tell(&self) -> usize {
  95         self.pos * 8 - (self.bits as usize)
  96     }
  97
  98     /// Reports the amount of bits left until the end of the bitstream.
  99     pub fn left(&self) -> isize {
 100         ((self.src.len() as isize) - (self.pos as isize)) * 8 + (self.bits as isize)
 101     }
 102
 103     fn fill32be(&mut self, src: &[u8]) {
 104         let nw = (u32::from(src[0]) << 24) |
 105                  (u32::from(src[1]) << 16) |
 106                  (u32::from(src[2]) <<  8) |
 107                  (u32::from(src[3]) <<  0);
 108         self.cache |= u64::from(nw) << (32 - self.bits);
 109     }
 110
 111     fn fill32le16(&mut self, src: &[u8]) {
 112         let nw = (u32::from(src[1]) << 24) |
 113                  (u32::from(src[0]) << 16) |
 114                  (u32::from(src[3]) <<  8) |
 115                  (u32::from(src[2]) <<  0);
 116         self.cache |= u64::from(nw) << (32 - self.bits);
 117     }
 118
 119     fn fill32le32(&mut self, src: &[u8], lsb: bool) {
 120         let nw = (u32::from(src[3]) << 24) |
 121                  (u32::from(src[2]) << 16) |
 122                  (u32::from(src[1]) <<  8) |
 123                  (u32::from(src[0]) <<  0);
 124         if lsb {
 125             self.cache |= u64::from(nw) << self.bits;
 126         } else {
 127             self.cache |= u64::from(nw) << (32 - self.bits);
 128         }
 129     }
 130
 131     #[inline(always)]
 132     fn refill(&mut self) -> BitReaderResult<()> {
 133         if self.pos >= self.src.len() { return Err(BitstreamEnd) }
 134         while self.bits <= 32 {
 135             if self.pos + 4 <= self.src.len() {
 136                 let buf = &self.src[self.pos..];
 137                 match self.mode {
 138                     BitReaderMode::BE      => self.fill32be  (buf),
 139                     BitReaderMode::LE16MSB => self.fill32le16(buf),
 140                     BitReaderMode::LE      => self.fill32le32(buf, true),
 141                     BitReaderMode::LE32MSB => self.fill32le32(buf, false),
 142                 }
 143                 self.pos  +=  4;
 144                 self.bits += 32;
 145             } else {
 146                 let mut buf: [u8; 4] = [0, 0, 0, 0];
 147                 let mut newbits: u8 = 0;
 148                 for out in buf.iter_mut().take(3) {
 149                     if self.pos < self.src.len() {
 150                         *out = self.src[self.pos];
 151                         self.pos += 1;
 152                         newbits += 8;
 153                     }
 154                 }
 155                 if newbits == 0 { break; }
 156                 match self.mode {
 157                     BitReaderMode::BE      => self.fill32be  (&buf),
 158                     BitReaderMode::LE16MSB => self.fill32le16(&buf),
 159                     BitReaderMode::LE      => self.fill32le32(&buf, true),
 160                     BitReaderMode::LE32MSB => self.fill32le32(&buf, false),
 161                 }
 162                 self.bits += newbits;
 163             }
 164         }
 165         Ok(())
 166     }
 167
 168     #[inline(always)]
 169     fn read_cache(&mut self, nbits: u8) -> u32 {
 170         let res = match self.mode {
 171             BitReaderMode::LE => ((1u64 << nbits) - 1) & self.cache,
 172             _                 => self.cache >> (64 - nbits),
 173         };
 174         res as u32
 175     }
 176
 177     fn read_cache_s(&mut self, nbits: u8) -> i32 {
 178         let res = match self.mode {
 179             BitReaderMode::LE => ((self.cache as i64) << (64 - nbits)) >> (64 - nbits),
 180             _                 => (self.cache as i64) >> (64 - nbits),
 181         };
 182         res as i32
 183     }
 184
 185     #[inline(always)]
 186     fn skip_cache(&mut self, nbits: u8) {
 187         match self.mode {
 188             BitReaderMode::LE => self.cache >>= nbits,
 189             _                 => self.cache <<= nbits,
 190         };
 191         self.bits -= nbits;
 192     }
 193
 194     #[inline(always)]
 195     fn reset_cache(&mut self) {
 196         self.bits = 0;
 197         self.cache = 0;
 198     }
 199
 200     /// Reads the specified amount of bits as an unsigned value.
 201     ///
 202     /// The amount should fit into 32 bits, if you need more then
 203     /// you should read it as several parts. If the amount of bits
 204     /// requested to read is larger than the amount of bits left the
 205     /// call will return [`BitstreamEnd`].
 206     ///
 207     /// # Examples
 208     ///
 209     /// ```
 210     /// use nihav_core::io::bitreader::{BitReader,BitReaderMode};
 211     ///
 212     /// # use nihav_core::io::bitreader::BitReaderResult;
 213     /// # fn foo() -> BitReaderResult<u32> {
 214     /// let bits: [u8; 4] = [ 42, 43, 44, 45 ];
 215     /// let mut br = BitReader::new(&bits, BitReaderMode::BE);
 216     /// let value = br.read(17)?;
 217     /// # Ok(value)
 218     /// # }
 219     /// ```
 220     ///
 221     /// [`BitstreamEnd`]: ./enum.BitReaderError.html#variant.BitstreamEnd
 222     #[inline(always)]
 223     pub fn read(&mut self, nbits: u8) -> BitReaderResult<u32> {
 224         if nbits == 0 { return Ok(0) }
 225         if nbits > 32 { return Err(TooManyBitsRequested) }
 226         if self.bits < nbits {
 227             if let Err(err) = self.refill() { return Err(err) }
 228             if self.bits < nbits { return Err(BitstreamEnd) }
 229         }
 230         let res = self.read_cache(nbits);
 231         self.skip_cache(nbits);
 232         Ok(res)
 233     }
 234
 235     /// Reads the specified amount of bits as a signed value.
 236     ///
 237     /// Beside signedness it behaves the same as [`read`].
 238     ///
 239     /// [`read`]: #method.read
 240     pub fn read_s(&mut self, nbits: u8) -> BitReaderResult<i32> {
 241         if nbits == 0 || nbits > 32 { return Err(TooManyBitsRequested) }
 242         if self.bits < nbits {
 243             if let Err(err) = self.refill() { return Err(err) }
 244             if self.bits < nbits { return Err(BitstreamEnd) }
 245         }
 246         let res = self.read_cache_s(nbits);
 247         self.skip_cache(nbits);
 248         Ok(res)
 249     }
 250
 251     /// Reads single bit from the stream and interprets it as a boolean value.
 252     #[inline(always)]
 253     pub fn read_bool(&mut self) -> BitReaderResult<bool> {
 254         if self.bits < 1 {
 255             if let Err(err) = self.refill() { return Err(err) }
 256             if self.bits < 1 { return Err(BitstreamEnd) }
 257         }
 258         let res = self.read_cache(1);
 259         self.skip_cache(1);
 260         Ok(res == 1)
 261     }
 262
 263     /// Retrieves the next bits from the stream without advancing.
 264     ///
 265     /// If the bitstream is shorter than the amount of bits requested the result is padded with zeroes.
 266     ///
 267     /// # Examples
 268     ///
 269     /// ```
 270     /// use nihav_core::io::bitreader::{BitReader,BitReaderMode};
 271     ///
 272     /// # use nihav_core::io::bitreader::BitReaderResult;
 273     /// # fn foo() -> BitReaderResult<u32> {
 274     /// let bits: [u8; 4] = [ 42, 43, 44, 45 ];
 275     /// let mut br = BitReader::new(&bits, BitReaderMode::BE);
 276     /// let peek_value = br.peek(8); // this should return 42
 277     /// let value = br.read(8)?; // also 42
 278     /// # Ok(value)
 279     /// # }
 280     /// ```
 281     #[inline(always)]
 282     pub fn peek(&mut self, nbits: u8) -> u32 {
 283         if nbits > 32 { return 0 }
 284         if self.bits < nbits { let _ = self.refill(); }
 285         self.read_cache(nbits)
 286     }
 287
 288     /// Skips the requested amount of bits.
 289     ///
 290     /// The amount of bits to skip can be arbitrary large.
 291     /// If it skips more bits than there are actually in the stream the call will return [`BitstreamEnd`]
 292     ///
 293     /// [`read`]: #method.read
 294     /// [`BitstreamEnd`]: ./enum.BitReaderError.html#variant.BitstreamEnd
 295     #[inline(always)]
 296     pub fn skip(&mut self, nbits: u32) -> BitReaderResult<()> {
 297         if u32::from(self.bits) >= nbits {
 298             self.skip_cache(nbits as u8);
 299             return Ok(());
 300         }
 301         let mut skip_bits = nbits - u32::from(self.bits);
 302         self.reset_cache();
 303         self.pos += ((skip_bits / 32) * 4) as usize;
 304         skip_bits &= 0x1F;
 305         self.refill()?;
 306         if skip_bits > 0 {
 307             self.skip_cache(skip_bits as u8);
 308         }
 309         Ok(())
 310     }
 311
 312     /// Seeks to the absolute bit position in the stream.
 313     /// If the requested position lies after the bitstream end the function returns [`TooManyBitsRequested`].
 314     ///
 315     /// # Examples
 316     ///
 317     /// ```
 318     /// use nihav_core::io::bitreader::{BitReader,BitReaderMode};
 319     ///
 320     /// # use nihav_core::io::bitreader::BitReaderResult;
 321     /// # fn foo() -> BitReaderResult<u32> {
 322     /// let bits: [u8; 4] = [ 42, 43, 44, 45 ];
 323     /// let mut br = BitReader::new(&bits, BitReaderMode::BE);
 324     /// br.seek(16)?;
 325     /// let value = br.read(8)?; // this should return 44
 326     /// # Ok(value)
 327     /// # }
 328     /// ```
 329     ///
 330     /// [`TooManyBitsRequested`]: ./enum.BitReaderError.html#variant.TooManyBitsRequested
 331     pub fn seek(&mut self, nbits: u32) -> BitReaderResult<()> {
 332         if ((nbits + 7) >> 3) as usize > self.src.len() { return Err(TooManyBitsRequested); }
 333         self.reset_cache();
 334         self.pos = ((nbits / 32) * 4) as usize;
 335         self.skip(nbits & 0x1F)
 336     }
 337
 338     /// Aligns the bit position to the next byte boundary. If already at byte boundary the function does nothing.
 339     ///
 340     /// # Examples
 341     ///
 342     /// ```
 343     /// use nihav_core::io::bitreader::{BitReader,BitReaderMode};
 344     ///
 345     /// # use nihav_core::io::bitreader::BitReaderResult;
 346     /// # fn foo() -> BitReaderResult<()> {
 347     /// let bits: [u8; 4] = [ 42, 43, 44, 45 ];
 348     /// let mut br = BitReader::new(&bits, BitReaderMode::BE);
 349     /// br.skip(17)?; // now reader is at bit position 17
 350     /// br.align(); // now reader is at bit position 24
 351     /// br.align(); // now reader is still at bit position 24
 352     /// # Ok(())
 353     /// # }
 354     /// ```
 355     pub fn align(&mut self) {
 356         let pos = self.bits & 7;
 357         if pos != 0 {
 358             self.skip_cache(pos);
 359         }
 360     }
 361 }
 362
 363 /// Returns a variable with `len` amount of low bits in reverse order.
 364 ///
 365 /// # Examples
 366 /// ```
 367 /// use nihav_core::io::bitreader::reverse_bits;
 368 /// reverse_bits(0b010101, 6); // the result should be 0b101010
 369 /// ```
 370 pub fn reverse_bits(inval: u32, len: u8) -> u32 {
 371     if len == 0 { return 0; }
 372     const REV_TAB: [u8; 16] = [
 373         0b0000, 0b1000, 0b0100, 0b1100, 0b0010, 0b1010, 0b0110, 0b1110,
 374         0b0001, 0b1001, 0b0101, 0b1101, 0b0011, 0b1011, 0b0111, 0b1111,
 375     ];
 376
 377     let mut ret = 0;
 378     let mut val = inval;
 379     for _ in 0..8 {
 380         ret = (ret << 4) | u32::from(REV_TAB[(val & 0xF) as usize]);
 381         val >>= 4;
 382     }
 383     ret >> (32 - len)
 384 }
 385
 386 #[cfg(test)]
 387 mod test {
 388     use super::*;
 389
 390     #[test]
 391     fn br_works() {
 392         const DATA: [u8; 18] = [0b00011011; 18];
 393         let src = &DATA;
 394         let mut br = BitReader::new(src, BitReaderMode::LE16MSB);
 395
 396         for _ in 0..8 {
 397             assert_eq!(br.read(16).unwrap(), 0x1B1B);
 398         }
 399         const DATA2: [u8; 1] = [ 0b00011011 ];
 400         let src = &DATA2;
 401         let mut br = BitReader::new(src, BitReaderMode::LE);
 402         assert_eq!(br.read_s(5).unwrap(), -5);
 403     }
 404 }